Genetics Not Always Winner In Struggle Against Obesity

Sometimes we receive letters from parents, both of whom have spent a lifetime unsuccessfully battling obesity. They want to spare their child the same fate, and ask if this is impossible.

The answer is a resounding no. We advise them to follow their pediatrician's directives carefully, learning to avoid both overfeeding and overrestriction. The latter can be extremely dangerous and lead to retarded growth.

These parents are right to be concerned. As most people who play the lottery know, it is hard to beat the odds. More than 20 years ago, one of us (Dr. Mayer) quantified the odds of having a weight problem if you were born to a household that had them. If both father and mother were thin, the child's chances of being overweight were less than 10 percent. With one parent overweight, the odds jumped to 40 percent. Having two overweight parents doubled the figure to 80 percent.

The riddle of genetics vs. environment has occupied scientists for years. Studies done with mice indicate that heredity is important to weight status. In one study, mice with a dominant gene for obesity grew fat while mice from the same litter but with a recessive gene for obesity maintained a normal weight.

Discovering the relationship between genetics and environment in humans is far more difficult, although studies of the weight status of twins have strongly supported the importance of genetics. A study appears to have helped quantify those effects. Dr. Albert Stunkard of the University of Pennsylvania and researchers from the Center for Demographic and Population Genetics of the University of Texas Health Sciences Center collaborated with Danish investigators. Their study compared the weight status of a large group of Danish adoptees, their biological and their adoptive parents.

Through the Danish Adoption Register and the Danish Folk Register, which contains the names and addresses of Denmark residents, it was possible to trace 4,643 men and women adopted between 1924 and 1927. This Adoption Register also provides information about both the adoptive and biological parents.

Seventy-nine percent of the adoptees responded to an initial health questionnaire, which included information about health and weight status. From that pool the researchers selected a total of 540 adults from both sexes and five age categories. They were divided into four weight classes, defined as thin, normal, overweight and obese.

A second questionnaire was then mailed to adoptees, both their biological and adoptive parents and their siblings to obtain information about height and weight. Checks of the information indicated that it was reliable.

The findings were striking. There was a clear-cut relationship between the weight class of the adoptees and the body-mass index of their biologic parents. This relationship was stronger between biological mothers and adoptees than between biological fathers and adoptees. (Body-mass index is a measure of body fatness that describes the relationship between height and weight in a single number.) However, there was no relationship between the adoptee's weight class and that of his adopted parents. This finding held true across all four weight classes.

Does this mean that some of us should just accept the fact that we are born to be fat? No. Laboratory studies have shown that environment can play a powerful role. In an editorial accompanying the report of the Danish study, obesity expert Dr. T.B. VanItallie cites two types of studies that underscore the effects of environment. In one, genetically non-obese rats can be fattened if offered a high-fat ''cafeteria'' diet of snack foods from the supermarket. Curiously while most gain weight, the amount varies, suggesting that genetic differences exist in the metabolic response to overeating.

In another study, genetically obese Zucker (''fatty'') mice, which will get fat on a diet of laboratory chow, became far fatter if fed the ''cafeteria'' diet. When returned to their lab diets, they lost some of the excess fatness. Evidence from human population studies shows that socioeconomic factors affect weight status, too. The second National Health and Nutrition Examination Survey, conducted between 1976 and 1980, found the prevalence of overweight to be higher among women below the poverty line than in those above it. (Overweight was somewhat more prevalent in men with income above the poverty level.)

The important message of these and other investigations that have demonstrated strong genetic influences on obesity is that they help identify a population ''at risk.'' Parents who ask for guidance show wisdom in recognizing a potential problem. Establishing sound eating habits right from the start and later encouraging a child to enjoy regular, vigorous physical activity are the keystones of an effective counteroffensive. Adopting that approach may help explain why 20 percent of the children in households where both parents are obese do not develop a weight problem.